The present invention is directed to decimation filters, and more particularly to a chain of finite impulse response (FIR) filters for decimating a digital signal with a high sampling frequency, down to a digital signal with a low sampling frequency, having a long word length while simultaneously filtering out line noise introduced by the line voltage.
In the application of material metering machines, load cells are relied upon to provide basic weight measurements. Typically, the material is loaded into a hopper, or a vessel, which is attached to the load cell. As the material passes out of the hopper at the bottom, new material is fed into the top of the hopper. The load cell monitors the weight of the hopper and passes those measurements to a processor which calculates the flow rates from the vessels. One area of technology where the material metering machines are used is for resinous or plastic materials. Typically, the resinous or plastic material is in the form of a pellet, of which there are approximately 30 to 80 to a gram. The pellets are passed though a weigh vessel one at a time, or at most a few at a time. Because the vessels have a high tare weight, which may be several thousand pounds, the weigh board must be able to monitor very minute changes in weight to insure the proper flow rate.
However, in this environment there are many sources of noise, both electrical and mechanical. For example, electrical noise may be introduced by converting the analog signal from the load cell to a digital signal that is used by a feedback loop to control the flow rate of the material through the vessel. Another source of electrical noise is line noise, which is the noise introduced into the system by the power supply.
Measurements of the resinous pellets flowing through the hoppers must be made with great accuracy. This is due to the fact that the resinous pellets are very light. Plastic blending machines, which use plastic or resinous pellets, require very precise measurements that are needed for low recipe ratios and run rates in the presence of noise and high tare weights.
To meet the strict requirements needed by plastic blending machines, special purpose digital signal processors (DSPs), which contain the signal processing and filtering function on a single chip are required to minimize the amount of noise introduced into the system. The single chip devices currently available offer limited filtering capabilities and are relatively expensive. Presently, the single chip DSPs do not provide filtering capabilities for the 50 Hz and 60 Hz line noise introduced into the system by power supply, which is a significant factor when carrying out the precise weight measurements needed for plastic blending machines.
Several methods are currently used to reduce the noise from material metering machines. One method uses active vibration control, wherein undesired noise or other vibrations are reduced or eliminated by adding an equal and opposite amount of noise or vibration. Active vibration control is only used to eliminate mechanical noise, such as the noise introduced by the moving parts of the machine. Additionally, active vibration control systems can be relatively expensive because the systems require specialized components and a great amount of processing power. Furthermore, active vibration control methods do not reduce the noise added to the system from the line voltage.
Another method to remove unwanted noise from machine metering systems uses adaptive notch filtering, also known as Adaptive Line Enhancement (ALE). Under the ALE method, vibrations are converted to a digital signal at a fixed frequency by an analog-to-digital (A/D) converter. The digital signal is then applied to a decimation filter to reduce the number of samples used in subsequent calculations. The decimation filter also provides some anti-aliasing filtering to smooth the signal. The decimated signal is then applied to an adaptive notch filter to enhance the signal by filtering most of the noise and harmonics. Although the ALE method filters out most of the unwanted noise and enhances the rejection characteristics of the filtration, the ALE method does not filter out the noise generated by the line voltage at the 50 Hz and 60 Hz frequencies, which can introduce errors in the flow rate of plastic blending machines. Furthermore, the ALE method requires additional processing power over conventional methods, which increases the cost of the system.
Thus, there is a need in the art for an inexpensive noise reduction method to reduce the noise, both electronic and mechanical, from material metering machines. There is a further need in the art for a noise reduction system for use with material metering machines to filter out line noise associated with the 50 Hz and 60 Hz frequencies to provide an output signal that has the stability to drive material metering machines that require precise measurements needed to achieve low mixing ratios in the presence of high tare weights.
The present invention meets the above-described needs in an apparatus for filtering a digital signal that is sampled at a very high frequency and generating a digital output signal with a very low sampling frequency that has also been filtered to remove line noise introduced by the apparatus"" power supply. Generally described, the apparatus first receives a digital signal, which has been sampled at a high frequency rate. The high frequency input signal is passed to a first decimation element, which contains at least one digital filter. The first decimation element removes the noise in the signal introduced by an analog-to-digital (A/D) converter. The first decimation element also conditions the signal by reducing the sampling frequency of the digital signal required by a second decimation element.
The second decimation element receives the digital signal from the first decimation element at a reduced sampling frequency. The second decimation element filters out noise introduced in the signal from the line voltage by passing the signal through a series of decimation filters. Each of the filters provides rejection of the 60 Hz line noise and its harmonics while simultaneously reducing the sampling frequency of the digital signal. The resultant output of the second decimation element is a digital signal having a long word length and sampled at a very low frequency. In an exemplary embodiment, the digital output signal is 20 bits wide and has a sampling frequency of 10 Hz, providing a decimation ratio of 120:1.
To achieve the decimation ratio of 120:1, the second decimation element contains a series of filters that each has a particular decimation ratio. For example, the second decimation element can contain three filters that reduce the sampling frequency of the signal from 1200 Hz to 10 Hz in a series of successive steps. The first filter in this example has a decimation ratio of 2:1, thereby reducing the sampling frequency of the signal from 1200 Hz to 600 Hz. The first filter however, has an averaging length of 20, which produces nulls at multiples of 30 Hz. The longer filter length filters out noise introduced by the line voltage at the 60 Hz frequency while providing the decimated signal sampled at 600 Hz.
The second filter in the second decimation element has a decimation ratio of 10:1. The second filter receives the output signal from the first filter at the sampling frequency of 600 Hz and outputs a digital signal sampled at 60 Hz. Instead of using a conventional filter of length 10, the second filter has an averaging length of 20, which produces nulls in the frequency response at multiples of 30 Hz, which further reduce the line noise in the signal while simultaneously creating null at the 60 Hz frequency and its harmonics.
The third filter in the second decimation element has a decimation ratio of 6:1 to reduce the sampling frequency of the digital signal from 60 Hz to 10 Hz. The third filter has a conventional length of 6, which provides the appropriate decimation ratio and produces nulls at integer multiples of 10 Hz to further filter out the line noise at 60 Hz and also at 50 Hz.
The apparatus can additionally contain a bank of selectable filters, which are used to filter out the machine noise from the material metering machine. Each filter in the filter bank has a sub-hertz 3-dB cutoff frequency. The selection of a particular filter is a function of the machine""s operating characteristics and is based on the closed loop performance during the operation of the material metering machine.
The apparatus can additionally contain a digital signal processor (DSP) unit that conditions the input signal from the weigh board associated with the material metering machine before it is applied to the decimation elements. The DSP receives an analog data signal from the weigh board attached to the material metering machine and conditions the signal by passing it through a differential filter to provide anti-aliasing filtering. The filtered analog signal is passed through an analog-to-digital (A/D) converter before being sent to the first decimation element.